Iron coating for refractory metal



United States Patent Ofifice 3,282,809 Patented Nov. 1, 1966 3,282,809 IRON COATING FOR REFRACTORY METAL Joseph E. Cemess, Cleveland, Milton L. Selker, Shaker Heights, and Betty Lee Berdan, Willowick, Ohio, assignors to Clevite Corporation, a corporation of Ohio No Drawing. Filed Nov. 7, 1962, Ser. No. 236,155 7 Claims. (Cl. 204-37) titanium metal surface so as to render it capable of withstanding normal frictional or sliding contact without excessive wear.

Attempts have been made to adapt titanium parts for applications involving frictional contact by applying metallic coatings possessing superior wearing properties. Coatings of chromium have been applied, for example, by electrolytic plating processes. In general, however, considerable difficulty is encountered in obtaining a satisfactory bond between the coating material and the titaninum base material. Additionally, a high cost coating material such as chromium even though successfully bonded involves the use of special lubricants, and in itself possesses less than optimum wear properties, particularly in the case of heavily loaded parts such as gears.

It is a principal object of this invention to provide an iron coating on a part fabricated from a refractory metal such as titanium or titanium alloy to provide a surface material which can be hardened by known techniques and on which low cost commercially available lubricants can be employed. The lubrication of iron alloys has been studied extensively for many years and practically all low cost commercially available lubricants are for iron materials. Additionally, iron can be hardened by many well-known techniques to obtain desirable wear properties. Thus, the invention contemplates the provision of an iron coating on a refractory metal such as titanium to combine the most desirable properties of the two metals.

Another more specific object of the invention is to coat a part fabricated from a refractory base metal such as titanium with iron by an electrolytic plating process and then to heat treat the part to establish a desired wear property and to strengthen the bond between the iron and base metal.

Other objects and advantages will become apparent from the following description which discloses a preferred embodiment of theinvention.

In general the invention resides in the realization that a refractory material such as titanium can be advantageously rendered useful in the fabrication of parts requiring a high strength-weight ratio and subjected to frictional wear by providing on the part surface an iron coating which can be heat treated to a predetermined hardness and lubricated with conventional low cost lubricants. We have successfully achieved an iron coating on parts fabricated from various titanium alloys by anodizing the'surface in a zinc fluoride bath prior to electroplating in a ferrous sulfate bath. Surface hardness of the iron and good adhesion of the iron to the titanium is augmented by subsequent heat treatment.

Referring now to the specific coating process employed, samples of commercial titanium and two different alloys (TiA, Ti6Al4V, and TiCl30AM) were utilized as test parts and fabricated to ball configurations suitable for testing in a Shell four-ball tester.

The test parts were first cleaned by application of a conventional degreasing agent such as trichloroethylene to remove existing surface films. The next step was to soak the part for two minutes in an etching solution composed of the following:

Percent (by volume) Nitric acid -c 20 Hydrofluoric acid 2 The combination of nitric and hydrofluoric acid in the etching solution was effective in removing the light oxidation film from the surface of the test parts very uniformly. Following this acid soak the test parts were thoroughly rinsed to completely remove the etching solution from the surface.

The next step was to anodize the test parts in a zinc fluoride bath for 20 minutes at a current density of 45 amperes per square foot. The bath consisted of:

Ethylene glycol cc 48 percent hydrofluoric acid cc 20 Zinc fluoride grams 10 Prior to insertion into the anodizing bath the test parts i were dipped for one-half minute into a container of ethylene glycol to thoroughly wet the surface of the parts and remove any water from the surface. The parts were placed in the anodizing bath with current off and removed Ferrous sulfate (FeSO -7H O) grams 300 Ferrous chloride (FeCl -4H O) do 42 Ammonium sulfate do 15 Sodium formate do 15 Boric acid do 35 Sodium lauryl sulfate do 1 Temperature F -440 pH 3.7-4.2 Anodes Armco ingot iron The parts were placed in the ferrous sulfate bath with current on, plated at a current density of ZOamperes per square foot for two'hours. The ammonium sulfate, sodium formate and boric acid comprised buffering agents in the bath While the sodium lauryl sulfate served as a wetting agent. The ferrous chloride was utilized to aug- -ment the conductivity of the solution.

After removal from the electroplating iron sulfate bath the test parts were heat treated by carburizing for .two hours at 1600 F. followed by an oil quench. The heat treatment served not only to provide a desired surface hardness of the iron coating but also to augment the bond between the iron and titanium. A sample thus heat treated was tested for wear properties in a conventional Shell four-ball tester. With a 50 kg. load the iron plated surface exhibited wear characteristics comparable to SAE 52100steel with SAE 30 oil. I

The iron plating process and subsequent car-burization resulted in an iron coating approximately 98 percent pure and having a thickness of 0.002 inch. A metallographic examination of the carburized specimen revealed the typical structure of carburized iron and-an intermediate phase between the iron and titanium about 0.5 mil in thickness. This phase Was insoluble in one part nitric acid and two parts water, and possessed exceptional wear properties.

It will be apparent that the iron thickness may be varied as desired by varying the electroplating time. The wear properties and hardness of the iron is similarly dependent on the heat treating process utilized. The strengthening of the bond between the iron and titanium by heat treating is not believed to be dependent on the particular carburization process described but is believed to be .elfected by various heat treating methods and thus not critical.

The described process accordingly results in improved light Weight parts composed of iron coated refractory metal possessing both the desirable strength-weight characteristics of the refractory metal and the wear and lubrication advantages of iron.

While there has been described what at present is believed to be a preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, to cover in the appended claims all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed and desired to be secured by United States Letters Patent is:

1. A part having a high strength-weight ratio for applications involving frictional wear composed of a base metal selected from the group consisting of titanium and alloys predominantly of titanium, a combined zinc and titanium anodized layer on said base metal, and an electroplated layer of iron superimposed on said .zinc titanium anodized layer, said layer of iron being carburized.

2. A part as set forth in claim 1, further characterized by said layer of iron being about .002" thick.

3. The method of coating the surface of a part composed of titanium or an alloy predominantly of titanium, which includes the steps of: in a bath containing zinc ions anodizing the surface of the part to form a layer of zinctitanium oxide, electroplating a layer of iron onto said zinc-titanium oxide layer, and heating said layer of iron to carburize the iron to form a hard surface layer.

4. The method set forth in claim 3, further characterized in that said part is anodized in a solution containing zinc fluoride.

5. The method as set forth in claim 4, further characterized in that said anodized part is electroplated in a solution containing ferrous sulfate.

6. The method as set forth in claim 5, further characterized by prior to the anodizing step cleaning the part in an acid etching solution to remove titanium-oxide film.

'7. The method as set forth in claim 6, further characterized by after the acid etch and prior to the anodizing treating the part in ethylene glycol to wet the surface and remove water from the surface of the part.

References Cited by the Examiner UNITED STATES PATENTS 1,978,112 10/1934 Malby 204-38 2,711,389 6/1955 Beach et al 204-48 2,745,800 5/1956 Poor 204-37 2,834,101 5/1958 Boam et a1. 204-48 2,918,415 12/1959 Schaer 204-48 2,934,480 4/ 1960 Slomin 204-56 2,946,728 7/1960 Foise 204-37 2,965,551 12/1960 Richard 204-56 2,999,799 9/1961 Saubestre 204-48 JOHN H. MACK, Primary Examiner.

MURRAY TILLMAN, Examiner.

L. G. WISE, W. VAN SIS-E, Assistant Examiners. 

1. A PART HAVING A HIGH STRENGTH-WEIGHT RATIO FOR APPLICATIONS INVOLVING FRICTIONAL WEAR COMPOSED OF A BASE METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM AND ALLOYS PREDOMINANTLY OF TITANIUM, A COMBINED ZINC AND TITANIUM ANODIZED LAYER ON SAID BASE METAL, AND AN ELECTROPLATED LAYER OF IRON SUPERIMPOSED ON SAID ZINC-TITANIUM ANODIZED LAYER, SAID LAYER OF IRON BENG CARBURIZED. 